Process for controlling yarn tension and threadline stability during high speed heat treating of the yarn

ABSTRACT

A process for heat treating yarn that includes the steps of feeding the yarn into a stream of heated gas, causing the gas to convey the yarn in a first path while heating it, and subsequently changing the direction of advance of the yarn from said first path to a second path that intersects said first path, is modified to improve threadline stability without increasing tension in the zone where the yarn path changes direction. The modification includes the steps of: interposing a surface in the first path and withdrawing the yarn from the proximity of the surface in the direction of said second path at a rate to provide a single sharp change in direction at said surface toward said second path.

United States Patent Hill et a1. [45] May 16, 1972 [54] PROCESS FOR CONTROLLING YARN 2,869,967 1/1959 Breen .57/140 TENSION AND THREADLINE 2,959,909 11/1950 Sutherland et a] ...28/72. 12 STABILITY DURING HIGH SPEED 3,105,349 10/1963 Palm et a1 ...28/72. 12 3,244,785 4/1966 Hollandsworth. ....264/l68 HEAT TREATING OF THE YARN 3,558,760 1/1971 015611 ..264/l71 Inventors: William Green Hill; Whorley William Maxey, both of Bassett, Va.

Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

Filed: Aug. 17, 1970 Appl. No.: 64,219

U.S. Cl ..28/72.l7, 28/7212, 57/34 B,

264/168, 264/171, 264/234, 264/342 Int. Cl. ..D0ld 5/22, B29c 25/00 Field of Search ..264/l68, 171, 234, 345;

References Cited UNITED STATES PATENTS Griset ..28/7l.12

Primary Examiner-Jay H. Woo Attorney-Howard P. West, Jr.

[57] ABSTRACT A process for heat treating yarn that includes the steps of feeding the yarn into a stream of heated gas, causing the gas to convey the yarn in a first path while heating it, and subsequently changing the direction of advance of the yarn from said first path to a second path that intersects said first path, is modified to improve threadline stability without increasing tension in the zone where the yarn path changes direction. The modification includes the steps of: interposing a surface in the first path and withdrawing the yarn from the proximity of the surface in the direction of said second path at a rate to provide a single sharp change in direction at said surface toward said second path.

9 Claims, 6 Drawing Figures PAIENTEDMAY 16 I972 3, 662,440

FIG3

INVENTORS WILLIAM GREEN HILL WHORLEY WILLIAM HAXEY BY W ATTORNEY PROCESS FOR CONTROLLING YARN TENSION AND THREADLINE STABILITY DURING HIGH SPEED HEAT TREATING OF THE YARN BACKGROUND OF THE INVENTION This invention relates to the heat-treatment of oriented, heat-shrinkable, thennoplastic yarns. More particularly, it relates to control of yarn tension and threadline stability during the heat-treating process.

Recently there have been developed yarns for stretch fabrics, particularly ladies sheer hosiery, composed of filaments each of which has at least two eccentrically disposed, longitudinally continuous thermoplastic-polymer components which undergo shrinkage to differing extents when heattreated. The components may be joined side-to-side or may be in eccentric sheath/core arrangement. U. S. Pat. No. 3,399,108 issued Aug. 27, 1968, to E. H. Olson, discloses the production of such coil-crimped, stretch yarns. As described in that patent, the undrawn composite yarns are drawn to develop potential heat-shrinkage differences between the components. The drawn yarns are then heated under very low yarn tension to effect the differing shrinkages and the resultant forces leading to coil-crimping. At this point, coiling of the individual filaments of the yarn may or may not be visually evident. The heat-treated yarn is then tensioned at least enough to straighten it and is wound onto packages in straight uncoiled form. After fabric formation (e.g., knitting) from the packaged yarn, a heat-treatment of the untensioned fabric redevelops the coiling of the filaments and imparts stretchability to the fabric.

Crimp potential (as hereinafter defined) of yarns produced as described depends strongly on yarn tension during heatshrinkage. The lower the tension during heat shrinkage, the higher is the eventual crimp potential (CP) of the yarn. This improvement continues right up to the point where yarn is fed to the heat-shrinkage zone faster than its shrinking can reduce its length to match the lower rate of yam-removal from the heat-shrinkage zone. This latter condition causes a breakdown in threadline stability. The closer the tension is to this point of instability without making the process uncontrollable, the higher is the crimp potential of the resultant yarn.

Among the known processes for heat treating yarn, heating the yarn by heat-transfer with a surrounding hot gas is preferred where maintaining low tension is important. Preferably, the heated gas (e.g., air) is passed concurrently with the yarn through a heating chamber at a high velocity. Not only does this increase the efficiency of heat-transfer, but also it tends to automatically center the yarn within the yarnpassage of the chamber, thus reducing the possibility of yarn contact with solid surfaces of the chamber which normally would lead to increased yarn tension.

Yarn exiting the heating chamber possesses momentum which must be dissipated before the yarn may undergo change in direction at a yam-guide, roll or the like. Otherwise, the change in direction (and accompanying change in the momentum vector) requires the application of tension to keep the yarn in its guide or on a roll. In order to avoid the application of excessive tension, the point where change in direction occurs is ordinarily displaced away from the exit of the heating chamber whereby passage through ambient air dissipates most of the yarn's momentum before the change in direction occurs. Change of direction may also occur in free space without using guides or rolls, this being commonly known as a rooster tail. The higher the yarn velocity, the longer must be the unsupported yam-length between the exit of the heating chamber and the point where direction changes in order to maintain yarn tension low enough to provide high CP.

Yarns of the type described have been prepared by feeding prepackaged undrawn yarn through a drawing zone and immediately thereafter through a low-tension heat-shrinking zone. Linear yarn velocity in such a process, expressed as the peripheral velocity of the draw rolls, is characteristically less than 1,000 yd./min. (914 m./min.). It is commercially desirable to couple the extrusion process with the drawing and heatshrinking steps, without intermediate packaging. Yarn velocities in such a coupled process are characteristically of the order of 3,000 to 4,000 yd./min. (2,740 to 3,660 m./min.). At these higher velocities, the unsupported yam-length in the critical low-tension zone required to obtain high CP values is great, usually equalling or exceeding 25 inches (63.5 cm.). Such long unsupported yam lengths result in threadline instability so severe that adjacent threadlines interact and cause frequent yam-breaks. When the unsupported yarn length is decreased sufficiently to overcome threadline instability, tension necessary to keep the yarn in a guide or on a roll becomes too high to impart adequate CF to the yarn. The present invention overcomes these problems.

SUMMARY OF THE INVENTION The process to which this invention provides an improvement includes the steps of feeding the yarn into a stream of heated gas passing through a heat treating chamber, causing the gas to convey the yarn in a first path while heating it, and subsequently changing the direction of advance of the yarn from said first path to a second path that intersects the first path. The improvement comprises the steps of interposing a threadline stabilizer surface in the first path and withdrawing the yarn from the proximity of the surface in the direction of the second path at a rate to provide a single sharp change in direction at the surface toward the second path. The stabilizer surface is interposed a distance of from 3 to 16 inches (7.6 to 40.6 cm.) downstream from the exit of the heat treating chamber so that the first yarn path impinges substantially perpendicularly on the surface.

When a yarn impinges substantially perpendicularly onto a surface, its momentum is believed to be dissipated so that it is instantaneously free to follow a new direction under a downstream tension so low that none is transmitted upstream of the point of impingement. 1f the yarn strikes a surface at an angle departing too far from the perpendicular, it bounces off the surface at an uncontrolled angle and further tension-increasing change in direction is required before it can follow the withdrawal direction.

The precision in perpendicularity required in this process increases with increasing yarn speeds. 1n the examples, the yarns impinge on the stabilizer surface adjusted as closely as possible to truly perpendicular, i.e., well within 1 S. As yarn speed decreases, the need for such substantially perpendicular impingement diminishes. Under some conditions at customary yarn speeds, e.g., less than 1,000 yd./min. departures as high as 15 are operable and are considered to be approximately perpendicular as required herein.

Adjusting the rate of withdrawal of the yarn from the stabilizer surface so that the yarn continuously undergoes a single sharp change in direction avoids any buildup or uncontrolled bouncing at the point of impingement. This not only stabilizes the threadline but also maintains threadline tension at a very low level to provide a high crimp potential.

A preferred process according to this invention includes diverting the exhaust gas of the heating chamber away from the threadline by passing the stream of exhaust gas over a Coanda surface which is proximate the threadline at the exit of the heating chamber and diverges curvilinearly from the threadline downstream of the exit. Also, in order to provide sufficient tension for yarn-stability on rolls used to forward yarn from the low-tension zone without increasing the tension in the low-tension zone, well-known snubbing devices are ordinarily interposed between the low-tension zone and the forwarding rolls.

The process works exceptionally well when the yarn is composed of filaments having at least two eccentrically disposed, longitudinally continuous, thermoplastic-polymer components of differing heat shrinkage.

FIG. 1 is a schematic representation of a preferred process according to this invention.

FIG. 2 is the end view of one shape of threadline stabilizer showing change in yarn direction at its surface.

FIG. 3 is a view taken as indicated by 3-3 of FIG. 2.

FIGS. 4 and 5, analagous to FIG. 2, show other shapes of the threadline stabilizer which may be used.

FIG. 6 shows an alternate compact process effective when the threadline stabilizer has a smoothly curved convex surface.'

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS With reference to FIG. 1, the sheath/core filaments 10 departing spinneret assembly 29 are converged to individual yarns 13 while being quenched in air in a conventional quenching chamber 12. Yarns 13 are lead separately around a feed-roll assembly 16, through a draw zone and in multiple wraps around a draw-roll assembly 14. In the 'draw zone (i.e., between rolls l4, 16) the draw point of the yarns is localized within jet device 15. A set of rolls 17 pulls the drawn yarns 13 from draw roll 14 to prevent backwrapping and to feed each yarn 13 into a treatment chamber 18 where very low yarn tension exits. The yarns 13 pass through chamber 18 in a first path 20 toward stabilizer surface 110 which is shown in the form of a tube with its longitudinal axis within the plane of the first path 20 of the advancing yarns 13. The yarns impinge the surface 110 and are withdrawn with a single sharp change in direction along a second path 30, past a set of snubbing pins 114 by means of withdrawal rolls 112. The yarns then advance through guide 116 to a conventional packaging means 118.

In chamber 18, the yarn is conveyed and heat-relaxed by a concurrent high-speed flow of air around the yarn. Attached to the exit of chamber 18 is a Coanda surface '19 diverging downwardly away from the yarn in the first path. In this way, high velocity exhaust air is stripped away from yarns 13 advancing downwardly in the first path, thus avoiding air turbulence in the subsequent threadline travel particularly in the second yarn path 30 between the threadline stabilizer 110 and snubbing pins 114. Snubbing pins 114 are interposed in. this system to isolate a very low-tension zone in the first and second yarn paths 20, 30 downstream and upstream of tube 110 from the higher tension provided by rolls 112. The speed of rolls 112 is adjusted so that yarns 13 impinging tube 110 do not accumulate on nor uncontrollably bounce from the surface of the tube but simply undergo a single sharp change in direction in advancing from first path 20 to second path 30.

FIGS. 2-5 show yarns l3, advancing in a plane as indicated by arrows, striking a surface perpendicularly and being withdrawn at an angle with respect to their feed direction (i.e., between the first and second paths of advance described above). The surface must be long enough to intercept all of the advancing yarns, (FIG. 3), but no other length restriction applies. The angle 6 is not critical. It is usually acute. When a flat bar (FIG. 4) or a concave surface (FIG. is employed, angle 0 is preferably small enough to avoid contact of yarns 13 with edges of the device.

For the tubes (or rods) of FIG. 2, angles greater than 90 may be used, in which case yarns l3 follow the surface for a ways before being withdrawn from it. Change in yarn direction at the point of impingement remains sharp without any buildup of yarn. This effect, in fact, makes possible a very compact arrangement for obtaining the advantages of this invention. This is illustrated by FIG. 6 wherein yarn l3 impinges substantially normally on cylindrical threadline stabilizer 110 then follows its surface for a ways. A snubbing device 200 is mounted so that its position can be adjusted along dashed path 201. Forwarding roll 112 withdraws yarn 13 from the low-tension zone. Adjusting the position of snubbing device 200 provides varying levels of snubbing, which may be selected to provide the desired crimp potential.

DEFINITIONS AND TEST PROCEDURES The term yarn" used herein defines a substantially endless fibrous structure for textile purposes. As such, it may be spun from short fibers or may be composed of one or more continuous filaments either twisted or untwisted.

A drawn yarn is one which subsequent to its formation by extrusion is extended greatly in length to cause molecular orientation along the axial direction of each filament.

Relative viscosity is the ratio of flow times of a polymer solution and of the solvent measured at 25 C. in a viscometer. In the examples, which exemplify the process of this invention as applied to the production of nylon sheath/core filaments, the solvent is formic acid/water (/10 parts by weight) and the solution is prepared by dissolving 5.5 grams of the polymer in 50 ml./25 C. of the solvent.

Shringage or heat shrinkage denotes the decrease in length of a drawn yarn caused by heating.

Yarn shrinkage" is the percentage decrease in length of a straight yarn per unit of its mass. Thus, if a unit mass of yarn has length L on entering a zone, shrinks in the zone and has a length L, on leaving the zone, yarn shrinkage (S) is computed according to Equation l s loo/ 1) i 4) Crimp potential (CF) and crimp shrinkage (CS) are fully defined in Example I.

EXAMPLE I Polyhexamethylene adipamide (6-6 nylon) flake having a relative viscosity (RV) of 49 is prepared in conventional manner. A nonisomorphous copolymer composed of 50 percent by weight polyhexamethylene adipamide (6-6 nylon) monomer units and 50 percent by weight polyhexamethylene sebacamide/undecanedioamide/dodecanedioamide (6- 10/6-11/6-12 nylon) monomer units is also prepared in conventional manner, the resultant 6-6/6-10/6-11/6-12 copolymer flake having an RV of 55. The mixture of sebacic, undecanedioic and dodecanedioic acids used in the copolymerization is composed of 37 percent by weight dodecanedioic acid and 63 percent by weight technical grade sebacic acid, the latter containing about 1 percent by weight undecanedioic acid and about 2 percent by weight dodecanedioic acid.

The two flakes (6-6 and 6-6/6-10/6-11/6-12) are formed into yarns composed of sheath/core filaments each of which has two eccentrically disposed, longitudinally continuous thermoplastic-polymer components. The process by which these sheath/core filaments are obtained is described by Hollandsworth in U. S. Pat. No. 3,244,785, issued Apr. 5, 1966. The spinneret assembly used is shown in FIG. 2 of the aforesaid patent and the sheath/core cross-section of each filament is as shown in FIG. 5 thereof. The 6-6 polymer forms the sheath and the 6-6/6-10/6-1 1 /66-1 2 polymer forms the core. Each filament is 48 percent by weight sheath and 52 percent core. The sheath, at its thinnest point, has a thickness equal to about 2 percent of the thickness of the filament. These yarns are spun, treated and packaged according to the process schematically illustrated in FIG. I.

In the draw-zone between rolls 14, 16, the yarns 13 are drawn 3.6X, the draw-point being localized within jet device 15 wherein a jet of air heated to 250 C. is directed onto the traveling yarn. This drawing process and the jet-device 15 are substantially as described by Pitzl in Example XI of U. S. Pat. No. 3,452,130, issued June 24, 1969. Another set of rolls 17 pulls the drawn yarns 13 from the draw-rolls 14 to prevent backwrapping and to feed each yarn 13 into a treatment chamber 18 where very low yam-tension exists. In chamber 18, the yarn 13 is heat-relaxed by a concurrent high-speed flow of air around each yarn 13, the temperature of the air being regulated to maintain C. at the exit of chamber 18. In this and the remaining examples, heated air is injected into chamber 18 at a constricted diameter point so that air velocity exceeds sonic velocity during a portion of its travel through chamber 18. Chamber 18 is substantially as described by Clendening in U. S. Pat. No. 3,261,07l.

nun-.-

Threadline stabilizer l (see FIGS. 2 and 3) is a metal tube about 2 in. (5 cm.) in diameter positioned with its longitudinal axis within the plane of advancing yarns 13 and perpendicular to their direction of advance. The distance between the exit of chamber 18 and the top of tube 110 (i.e., the unsupported yarn length) is about 10 in. cm.). Withdrawal rolls 112 remove yarns 13 from tube 110. The speed of rolls 112 is adjusted so that yarns 13, impinging on tube 110 do not accumulate on its surface but simply undergo a sharp change in direction at the surface of thread-line stabilizer 110. While the angle between these two yam-paths is not critical, it is about 60 in this case. Each yarn 13 is led by a suitable guide 116 to conventional packaging means 118, yarn speed at windup being about 2,600 yd./min. (2,377 m./min.). Peripheral velocity of roll 112 is 18 percent less than that of rolls 17, Le, yarn letdown in the low tension zone is 18 percent. Throughout the low-tension zone, yarns 13 are perfectly stable even though undergoing sharp change in direction only 10 in. (25 cm.) away from chamber 18.

Each crimpable seven-filament yarn 13, as packaged, has a total denier of 45. A skein of yarn 13 is prepared by winding 16 loops to provide an approximately 1,500 total-denier skein about 55 cm. long when suspended with a 500 gm. weight attached. After hanging about 1 minute, skein-length, L,, is measured. The 500 gm. weight is replaced with a 1.8 gm. weight (about 0.0012 grams/denier). The skein is subjected to steam at 250 F. 121 C.) in a conventional boarding oven for one minute and then air-dried for -60 minutes. Skein length, L is measured with the 1.8 gm. weight still attached. Skein length, L is measured immediately after replacing the 1.8 gm. weight with the 500 gm. weight. Crimp potential (CP) is defined and computed by Equation (2):

a 2/ 2 X 100 Crimp shrinkage (CS) is defined and computed by Equation 3 CS (L,L /L X 100 (3) The yarn of this example is characterized by CP 24.1% and C5 6.8%.

By comparison, yarn produced in the same manner except for reducing letdown to 16 percent in the low-tension zone (by increasing withdrawal rate) has a much lower crimp potential (12.6 percent) and a higher crimp shrinkage (7.9 percent). The importance of providing the minimum possible yam-tension in the low-tension zone is clearly evident.

EXAMPLE II Example I is repeated except that threadline stabilizer 1 10 is repositioned so its surface is only 4 in. (10.2 cm.) below the exit of chamber 18. Crimp potential and crimp shrinkage of the packaged yarn are substantially identical to those of Example I. At this position, however. a transient momentary accumulation of yarn 13 occasionally occurs on the surface of tube 110. Such accumulation sometimes causes tangles along the length of the yarn and the tangles cause poor running performance during knitting.

EXAMPLE III Example I is repeated except for repositioning tube 110 to a point about 16 in. (41 cm.) below the exit of chamber 18. Crimp potential and crimp shrinkage are unchanged. While operation is stable at this position, it is not as stable as in Example potential interference between adjacent yarns being indicated.

EXAMPLE IV Example I is repeated except for increasing air temperature in chamber 18 by about 20 C. and for increasing yarn letdown from 18 percent to 22 percent. Crimp potential increases to 33.9 percent and crimp shrinkage decreases to 4.9 percent. Threadline stability is adequate for continuous trouble-free operation. It is believed, however, that 22 percent is very close to the maximum letdown usable under these conditions without feeding yarn to the tube 110 faster than it will be withdrawn by downstream rolls 112. Such a condition would result in uncontrolled threadline instability.

EXAMPLE V Example I is repeated except that tubes having the following diameters are tested:

inches (cm.) 0.5 (1.27) 0.75 (1.90) 1.0 (2.54) 1.25 (3.17)

All are found to be equivalent to the 2 inch (5.0 cm.) tube. The smallest-diameter tube is difficult to position well due to slight deviations of individual yarns from the plane of yarns impinging upon it. Thus, a tube-diameter of about 0.5 inch 1.27 cm.) represents a lower practical limit in this dimension.

EXAMPLE Vl Example V is duplicated except for replacing the tube 110 with flat surfaced threadline stabilizers (i.e., metal bars) of the following widths, W.

inches (cm.) 0.5 (1.27) 1 (2.54) 2 (5.08) 3 (7.62) 5 (12.70) 12 (30.48)

Performance in each case is equivalent to that obtained in Example I. It is found that positioning of flat surfaces substantially normal to the advancing yarns is somewhat less critical than with curved surfaces, making flat surfaces preferred. Maximum operable width is unlimited, but for most installations one prefers a bar narrow enough to readily fit into available space. The minimum width is that on which all yarns can impinge simultaneously. While bars less than 0.5 in. (1.27 cm.) wide can be employed, there is ordinarily no benefit derived from further miniaturization.

EXAMPLE VII Example I is repeated except for replacing tube 110 with a circularly concave surface (FIG. 5) with a 6 inch (15.2 cm.) radius of curvature and a 12 inch (30.4 cm.) width, W, measured along the curved surface. Operation and results are substantially identical to Example I when this surface is centered with respect to the plane of advancing yarns and oriented to provide substantially normal impingement by the yarns.

EXAMPLE VIII This example follows the teaching of Example I except for complete removal of tube 110. Each yarn is allowed to change direction freely in air, forming what is commonly known as a rooster tail. In order to obtain crimp potential equivalent to that obtained in Example I, a rooster tail at least 27 in. (68.6 cm.) long is required. The yarns under these conditions are quite unstable, frequently contacting one another to cause breaks. Instability leading to yarn failures is unacceptable in such a high-speed multiple-yam process.

EXAMPLE IX Example VIII is repeated except that the stabilizer tube 110 of Example I is placed below the rooster tail and progressively raised toward the exit of chamber 18. Marginally stable performance results with all yarns touching the surface at a distance of about 27 in. (68 cm.). As tube 110 is raised to further shorten the free length of each yarn, stability progressively improves until the tube 110 is only 3-4 inches (7.5 to 10 cm.) from the exit of chamber 18, at which point accumulation of yarn on the surface begins to occur. Repetition of this test with all the shapes and sizes of stabilizer device described in Examples V, Vl and Vll reproduces these results.

EXAMPLE X Each of the previous examples describes a preferred process wherein a Coanda surface 19 of about 2.5 inch (6.4 cm.) wide plate curved into a cylindrical sector of about 4.0 inch (10.2 cm.) radius is attached in conventional manner to the exit surface of chamber 18. It is fastened adjacent to and parallel to the row of spaced yarns l3 exiting chamber 18. This example repeats the general process of the previous examples illustrating in four tests the effect of removing the Coanda surface with or without the use of a threadline stabilizer 110. Results are shown in the table below. Speed of yarn withdrawal is identical in each case at approximately 18 percent less than the constant peripheral speed of rolls 17; no attempt is made to maximize CP in each test. The threadline stabilizer is a flatsurfaced bar about 1.0 inch (2.5 cm.) wide.

In Test 1, using neither a Coanda surface nor a stabilizer surface, the unsupported yarn length is from 25 to 31 inches (63.5 to 78.7 cm.).Threadlines are very unstable, contact one another often and are frequently broken. Operation is unacceptably unstable.

Test 2 differs from Test 1, only in using the Coanda surface. Threadline stability is improved because the required length of unsupported yarn is reduced to within 12 to 23 inches (30.5 to 58.4 cm.). Residual instability and the resultant frequency of yarn breaks are still unacceptable.

Test 3, representing a preferred process of this invention, employs both a Coanda surface and a stabilizer surface. The stabilizer bar is positioned about 8 inches (20.3 cm.) below the exit of chamber 18. Threadlines are perfectly stable with no interaction and no resultant yarn breakage.

Test 4 repeats Test 3, except that the Coanda surface is removed. Threadline stability is seen to deteriorate some, but operation and continuity are quite acceptable. Thus, it is shown that the major factor in improving threadline stability is the use of the stabilizer surface. While the additional use of a Coanda surface is preferred, it is not required.

Throughout the foregoing discussion, yarn tension in the region just downstream of the heat-shrinkage zone has been described simply as low." As indicated, it is preferably about as low as it can be without the feed speed exceeding the withdrawal speed by more than the available yam-shrinkage. It is so low as to be unmeasurable with customary devices used to measure yam-tension. Moreover, it is so low that a tension device cannot be inserted without completely upsetting the operating balance of tensions. in the examples, each filament of the yarn at the low tension has the physical form of an extended coil-spring. It is inferred from this observation, and from knowledge of tensions which must be overcome in fabrics in order that the filaments may subsequently become tightly coiled, that the low tensions are between 0.005 and 0.5 gm. and preferably 0.05 gm. or less. For the yarn of the examples, the tension employed is believed to be about 0.001 gm./den.

Surfaces for use as threadline stabilizers are usually solid surfaces, most often of a metal. Glass, paper, wood, plastic and other solid surfaces are equally effective. Also effective are surfaces of liquids (e.g., mercury or water pools). While threadline stabilizers of only simple cross-sectional shape have been shown, it is apparent that stabilizers of more complex cross-sections can be employed with equal success.

Throughout this specification, the process has been described as involving impingement of a yarn onto the surface of a threadline stabilizer. It is believed that actual physical impingement may not always occur. Photographic studies of the point of impingement indicate that under some circumstances the required dissipation of momentum occurs in the thin layer of air next to the surface of the threadline stabilizer. This is difficult to confirm. In any event, for the purposes of this invention, the thin layer of stagnant air at the surface of the threadline stabilizer may be considered an extension of that surface.

In the examples the gas employed in the heating chamber is air which on first contacting the yarn exceeds sonic velocity. Air is preferred when it has no deleterious effect on the yarn because it is inexpensive. Any of the well-known permanent gases or steam can be used and sometimes more inert gases (e.g., nitrogen or carbon dioxide) will be preferred. It has also been observed that contacting the yarn with air at a velocity less than sonic gives satisfactory results.

The process of this invention provides needed yarn stability in zones of very low tension by permitting drastic shortening of the unsupported length of yarn. As described, this permits operation at exceedingly high yamspeeds. Its effectiveness, however, is not limited to high yarn-speeds; and, due to its simplicity, is desirable at more conventional processing speeds such as are obtained, for example, in commercial drawtwisters.

The examples are illustrative of a critical high-speed process wherein exceedingly low tensions are required in order to pre-crimp" a yarn of eccentric bicomponent filaments. This heat-shrinkage process is, of course, equally applicable to the development of other kinds of crimp or to the heat-shrinkage of yarns which are and remain straight.

Spun, monofilament and multifilament yarns may be processed according to this invention. The process is applicable to the treatment of a single yarn, but it is most advantageous when used to treat a plurality of parallel, closely spaced yarns. The resultant threadline stability eliminates yam-to-yam contacts and the subsequent snarling, breaking and/or longitudinal nonuniformities of the yarns.

It is apparent that many changes and modifications of the disclosed process may be made without departing from the spirit of the present invention which is accordingly, intended to be limited only by the scope of the appended claims.

What is claimed is:

1. In a process for heat treating yarn that includes the steps of feeding the yarn into a stream of heated gas at a velocity greater than 1,000 yards per minute, causing the gas to convey the yarn in a first path while heating it and changing the direction of advance of the yarn from said first path to a second path that intersects said first path, the improvement comprising the steps of: interposing a surface in said first path approximately perpendicular thereto and withdrawing said yarn from the proximity of said surface in the direction of said second path at a rate to provide a single sharp change of direction at said surface toward said second path.

2. The process as defined in claim 1, including the additional step of diverting a part of said gas stream from said first path prior to interposing said surface.

3. The process as defined in claim 1, said yarn being fed into said stream at a rate greater than the rate at which it approaches said surface.

4. The process as defined in claim 1, said yarn being a bicomponent differentially heat shrinkable yarn.

5. In a process for heat treating a heat shrinkable yarn that includes the steps of feeding the yarn at a velocity of from about 3,000 to about 4,000 yards per minute to a heating chamber having a stream of heated gas passing therethrough, causing the gas to impart momentum to and heat the yarn while conveying it from an entrance to an exit of said chamber and into a first path beyond said exit and changing the direction of the advance of said yarn from said first path to a second path that angularly intersects said first path, the improvement comprising: diverting a part of said stream from said first path at said exit; interposing a surface substantially perpendicular to said first path below said exit for dissipating yarn momentum; and withdrawing said yam from the proximity of said surface in the direction of said second path at a rate to provide a single sharp change in direction at said surface toward said second path.

6. The process as defined in claim 5, said surface being in- 

1. In a process for heat treating yarn that includes the steps of feeding the yarn into a stream of heated gas at a velocity greater than 1,000 yards per minute, causing the gas to convey the yarn in a first path while heating it and changing the direction of advance of the yarn from said first path to a second path that intersects said first path, the improvement comprising the steps of: interposing a surface in said first path approximately perpendicular thereto and withdrawing said yarn from the proximity of said surface in the direction of said second path at a rate to provide a single sharp change of direction at said surface toward said second path.
 2. The process as defined in claim 1, including the additional step of diverting a part of said gas stream from said first path prior to interposing said surface.
 3. The process as defined in claim 1, said yarn being fed into said stream at a rate greater than the rate at which it approaches said surface.
 4. The process as defined in claim 1, said yarn being a bicomponent differentially heat shrinkable yarn.
 5. In a process for heat treating a heat shrinkable yarn that includes the steps of feeding the yarn at a velocity of from about 3,000 to about 4,000 yards per minute to a heating chamber having a stream of heated gas passing therethrough, causing the gas to impart momentum to and heat the yarn while conveying it from an entrance to an exit of said chamber and into a first path beyond said exit and changing the direction of the advance of said yarn from said first path to a second path that angularly intersects said first path, the improvement comprising: diverting a part of said stream from said first path at said exit; interposing a surface substantially perpendicular to said first path below said exit for dissipating yarn momentum; and withdrawing said yarn from the proximity of said surface in the direction of said second path at a rate to provide a single sharp change in direction at said surface toward said second path.
 6. The process as defined in claim 5, said surface being interposed a distance of from 3 to 16 inches from said exit.
 7. The process as defined in claim 5, there being a plurality of said yarns being conveyed in substantially parallel paths toward said surface.
 8. The process as defined in claim 6, said yarns being fed into said chamber at a rate greater than the rate at which they approach said surface.
 9. The process as defined in claim 5, said yarn being a bicomponent differentially heat-shrinkable nylon yarn. 